Stats 101 for journos or why CFCs aren't the culprit

A study into the impacts of CFCs on global warming has been covered in depth by sections of Australia's media. But there's one major problem with the research: correlation doesn't equate to causation.

I never really enjoyed statistics classes at university but it was a compulsory subject and so I had no other choice. While I’ve since found Microsoft Excel can do all the hard work I was forced to learn to do manually, there’s one thing it can’t cover off that I was taught about in Quantitative Methods 1 – correlation doesn’t necessarily equate to causation.

To explain this principle with an example, in studying a range of sales data you find ice cream sales go up at the same time as beer sales during summer. Just because they happen to both go up at the same time it doesn’t therefore mean that eating ice cream is making these people become incredibly thirsty for a beer (although I do think there is a legitimate causal link between beer and souvlaki/kebab sales).

If you thought this was the case however, to be thorough you’d want to test the relationship by physical observation of the people eating ice cream to see what they did after consumption. You’d then realise that the ice cream was in fact being eaten by children under the age of 12 who don’t tend to drink much beer. And the beer drinkers weren’t eating all that much ice cream, but did seem to have an odd fetish for souvlakis at 2am in the morning.

This argument that CFCs are the real culprit is based on a new paper by Qing-Bin Lu published in the peer reviewed, International Journal of Modern Physics B. This paper finds that the concentration of CFCs in the atmosphere and changes in cosmic rays from the sun can be better correlated via some creative maths to changes in surface temperatures over the past century than CO2. Therefore global warming is about to end because CFCs are now being phased out.

The interesting thing is that this is extremely similar to arguments he ran in earlier papers published back in 2009 and 2010. There are good reasons for why these papers didn’t make an impact on scientific thought back then and why this latest paper will suffer the same fate – because he’s failed to adequately explain causality, and the correlation is also dodgy.

A number of CFCs, in addition to causing the hole in the ozone layer, are also powerful greenhouse gases so there’s some causal link at work here. But they aren’t the only gas in the atmosphere that we know traps heat. Greenhouse gases act to trap heat by absorbing particular wavelengths of energy, in particular thermal infrared, preventing them escaping into space. The interesting thing is each gas has its own characteristic set of wavelengths that it will absorb which can help us in identifying and measuring their respective impact.

Satellites from NASA and Japan have been measuring outgoing wavelengths of radiation from Earth now for several decades. These measurements (illustrated below) show that there has indeed been a reduction in the outgoing radiation at the wavelengths absorbed by CFCs over this time. But so have those wavelengths absorbed by CO2 and CH4 (methane).

Change in outgoing spectrum from 1970 to 1996 due to trace gases

This has been backed by other studies looking at surface measurement of different wavelengths of infrared energy. So if CFCs are solely to blame, then what is happening to all that infrared radiation being reflected back to earth by CO2 and CH4? Lu doesn’t address this.

“The physics of how CFCs might impose such a strong radiative forcing are not addressed. Lu mentions that the radiative forcing from CFCs haven't been directly measured, then moves onto statistical correlations. In fact, the greenhouse effect from CFCs have been quantified from surface observations of the infrared radiation spectrum (Evans 2006). The observed results are broadly consistent with model predictions [of the IPCC] of greenhouse forcing.”

When Lu made similar claims in a 2010 paper it was pulled to pieces in a